1. Field of the Invention
Embodiments of the present invention generally relate to a variable capacitor for radio frequency (RF) and microwave applications.
2. Description of the Related Art
As the size of semiconductors continues to shrink, so does the micro-electromechanical systems (MEMS) that are coupled to the semiconductors. MEMS devices may be used in miniature relay switches, capacitance switches, non-volatile memory elements and for many more applications. The MEMS devices have a suspended structure that moves between at least two positions to modify the electrical impedance to the flow of continuous or alternate current.
The MEMS devices may be built over complementary metal oxide semiconductor (CMOS) devices. MEMS devices are made using similar processing steps to those found in semiconductor foundries and therefore can be manufactured cost effectively on a wafer scale. Some of the issues that arise in MEMS devices include unwanted capacitive coupling, series inductance and losses. The MEMS devices may be disposed in cell or bitcells to collectively form a DVC. A DVC may be controlled in a binary fashion to generate an RF capacitance ranging from Cmin to Cmax. Many small MEMS switches may be combined in one cavity which are all actuated at the same time. Bitgroups are generated by either combining multiple cells, with e.g. 2×, 4×, 8×, etc. . . . the number of switches, or by using partial cells, e.g. with only ½, ¼ or ⅛ the number of switches in the cell.
The capacitance of the DVC may be customized to have a specific capacitance. To have a customized capacitance, the bitcells may be custom made to achieve the desired capacitance. One manner of obtaining the desired capacitance is to custom design each cell having only as many switches as required to generate the required RF capacitance, e.g. these partial cells contained only ½, ¼, ⅛, etc. . . . of the number of switches compared to a full-cell. The capacitive load of the control lines of these partial cells then is also scaled down proportionally compared to the standard cells.
Typically a large-value isolation resistor is required between the control-electrodes and the CMOS driver to make sure that the control-electrodes are RF-floating, which ensures that the RF currents don't flow into the CMOS driver which would hurt the Q. In order to achieve this, the impedance of this isolation resistor must be several orders larger than the impedance of the control-electrode to the moveable MEMS element over the whole RF frequency range (0.5 . . . 3.5 GHz). Typical values of the isolation resistor range from 100 kOhm to 10 MOhm. Because these partial cells have a lower capacitance between the control-electrode and the moveable MEMS element, a larger value of the isolation resistor is needed to achieve the same RF performance. These increased isolation resistors exhibit more parasitic capacitances, which makes it hard for the CMOS control circuit to match the dynamic performance of these partial cells to the full cells.
Therefore, there is a need in the art for a DVC having a desired capacitance without parasitic capacitances.